FIG. 3 of Japanese patent laid-open publication No. 9-36685 or FIG. 3 of the corresponding U.S. Pat. No. 5,754,666 discloses an audio signal reproduction apparatus according to a prior art for adding a signal having a frequency spectrum above an upper limit of higher frequency range of a reproduction frequency band or an upper limit of higher frequency range of an audio frequency band to an analog audio reproduced signal. FIG. 16 shows a configuration of the audio signal reproduction apparatus. Referring to FIG. 16, the audio signal reproduction apparatus is constituted by including a buffer amplifier 91, a filter circuit 92, an amplifier 93, a detector circuit 94, a time constant circuit 95, a noise generator 96, a filter circuit 97, a multiplier 98, and an adder 99.
First of all, an audio signal is inputted to the buffer amplifier 91 through an input terminal T1, and then distributed into two signals. One of distributed audio signals is inputted to the adder 99 as it is, and another distributed audio signal is inputted to the filter circuit 92, which is either a high-pass filter or a bandpass filter. The filter circuit 92 bandpass-filters an inputted audio signal, passes only a signal fallen within a specific band, and outputs a resultant signal to the amplifier 93. The amplifier 93 amplifies an inputted audio signal to a predetermined appropriate level, and outputs an amplified audio signal to the detector circuit 94, which includes the time constant circuit 95. The detector circuit 94 detects, for example, an envelope of an inputted audio signal so as to detect an envelope level of the inputted audio signal, and outputs a level signal indicating a detected envelope level to the multiplier 98 as a level control signal for adjusting a level of a noise component added to the original audio signal.
On the other hand, a noise component generated by the noise generator 96 is inputted to the filter circuit 97, which is either a high-pass filter or a bandpass filter. The filter circuit 97 passes a noise component fallen within a frequency band equal to or higher than 20 kHz, and outputs a resultant noise component to the multiplier 98. The multiplier 98 multiplies an inputted noise component by the level control signal outputted from the detector circuit 94 so as to generate a noise component having such a level that is proportional to a level indicated by the level control signal, and outputs a generated noise component to the adder 99.
Further, the adder 99 adds a noise component from the multiplier 98 to the original audio signal from the buffer amplifier 91, generates an audio signal to which the noise component is added, and outputs a noise component-added audio signal from an output terminal T2. In this case, a time constant of the time constant circuit 95 is selected to be equal to a predetermined value so that the noise component generated by the noise generator 96 can be matched to human auditory characteristics and an effect of improvement in sound quality of the audio signal can be increased.